This invention concerns a method for forming organized arrays of metal, alloy, semiconductor and/or magnetic clusters for use in the manufacture of electronic devices, such as high-density memory storage and nanoelectronic devices.
Fundamentally new technologies are required to continue increasing device integration density and speed. Conventional metal-oxide semiconductor-field-effect transistors soon will reach fundamental density and speed limits as a result of quantum mechanical tunneling. In order to scale electronic device sizes down to nanometer dimensions, systems containing increasingly fewer numbers of particles must be considered.
The ultimate limit is a system in which the transfer of a single charge quanta corresponds to information transfer or some type of logic operation. Such single-electron systems are presently the focus of intense research activity. See, for example, Single Charge Tunneling, Coulomb Blockade Phenomena in Nanostructure, edited by H. Grabert and M. H. Devoret, NATO ASI Series B: Physics Vol. 294 (1992). These systems have potential application to nanoelectronic circuits that have integration densities far exceeding those of present day semiconductor technology. See, Quantum Transport in Ultrasmall Devices, edited by D. K. Ferry, H. L. Grubin, C. Jacoboni, and A. Jauho, NATO ASI Series B: Physics Vol. 342 (1995).
Single-electron transistors based on the concept of Coulomb blockade are one proposed technology for realizing ultra-dense circuits. K. K. Likharev, Single-electron Transistors. Electrostatic Analogs of the DC SQUIDS,xe2x80x9d IEEE Trans. Magn. 23:1142 (1987); and IBM J. Res. Dev. 32:144 (1988). Coulomb blockade is the suppression of single-electron tunneling into metallic or semiconductor islands. In order to achieve Coulomb blockade, the charging energy of an island must greatly exceed the thermal energy. To reduce quantum fluctuations the tunneling resistance to the island should be greater than the resistance quantum h/e2. Coulomb blockade itself may be the basis of conventional logic elements, such as inverters. Id.
Equally promising is the fact that the Coulomb blockade effect can be used to pump charges one-by-one through a chain of dots to realize a frequency-controlled current source in which the current is exactly equal to I=ef, whereof is the clocking frequency. See, L. J. Geerligs et al., Frequency-locked Turnstile Device for Single-electrons, Phys. Rev. Lett., 64:2691 (1990); and H. Pothier et al., Single-Electron Pump Based on Charging Effects, Europhys. Lett. 17:249 (1992). Such turnstile devices are of fundamental interest as highly accurate current standards.
The clocking of charge through an array is also one model of information storage. It is possible that computation may be based on switching of currents rather than charge which, due to the extreme accuracy of single-electron current sources, may be more robust towards unwanted fluctuations than single-electron transistor-based circuits.
One of the most promising technologies for realizing terabyte memories is founded on the principle of the Coulomb blockade. Yano et al. have demonstrated room temperature operation of single-electron devices based on silicon nanocrystals embedded in SiO2. K. Yano et al., Room-Temperature Single-electron Memory, IEEE Trans. Electron. Devices, 41:1628 (1994); and K. Yano et al., Transport Characteristics of Polycrystalline-Silicon Wire Influenced by Single-electron Charging at Room Temperature, Appl. Phys. Lett., 67:828 (1995). Recently, a fully integrated 8xc3x978 memory array using this technology has been reported. K. Yano et al., Single-Electron-Memory Integrated Circuit for Giga-to-Tera Bit Storage, IEEE International Solid State Circuits Conference, p. 266-267 (1996).
Microelectronic devices based on the principle of Coulomb blockade have been proposed as a new approach to realizing electronic circuits or memory densities that go beyond the predicted scaling limit for present day semiconductor technology. While the operation of Coulomb blockade devices has been demonstrated, most operate only at greatly reduced temperatures and require sophisticated nanofabrication procedures. The size scales necessary for Coulomb blockade effects at such relatively elevated temperatures of about room temperature impose limits on the number, uniformity and connectivity of quantum dots. As a result, alternative methodologies of nanofabrication need to be investigated and developed.
The present invention provides a new process for making arrays comprising metal, alloy, semiconductor and/or magnetic clusters. An xe2x80x9carrayxe2x80x9d can be any arrangement of plural such clusters that is useful for forming electronic devices. Three primary examples of uses for such arrays are (1) electronic circuits, (2) arrangements of computer memory elements, both of which can be in one or several planes, and (3) sensors.
xe2x80x9cClustersxe2x80x9d as used herein refers to more than one, and typically three or more, metal, alloy, semiconductor or magnetic atoms coupled to one another by metal-type bonds or ionic bonds. Clusters are intermediate in size between single atoms and colloidal materials. Clusters made in accordance with the present invention also are referred to herein as xe2x80x9cnanoparticles.xe2x80x9d This indicates that the radius of each such cluster is on the order of about one nanometer. A primary goal of the present invention is to provide electronic devices that operate at or about room temperature. This is possible if the cluster size is made small enough to meet Coulomb blockade charging energy requirements at room temperature. While cluster size itself is not dispositive of whether the clusters are useful for forming devices operable at or about room temperature, cluster size is nonetheless quite important. It currently is believed that clusters having radii much larger than about two nanometers likely will not be useful for forming electronic devices that operate at or about room temperature.
The metal, alloy, semiconductor and/or magnetic clusters are bonded to xe2x80x9cscaffoldsxe2x80x9d to organize the clusters into arrays. xe2x80x9cScaffoldsxe2x80x9d are any molecules, including polymers, that can be placed on a substrate in predetermined patterns, such as linear bridges between electrodes, and to which clusters can be bonded to provide organized cluster arrays. Without limitation, scaffolds include biomolecules, such as polynucleotides, polypeptides, and mixtures thereof. Polypeptides capable of forming xcex1-helices are particularly useful scaffold-forming molecules. Polypeptides that are capable of forming other secondary structures, such as 310-helices, xcfx80-helices, and xcex2-sheets may in certain embodiments serve as scaffolds. Polypeptides that are capable of forming repetitive higher order structures (i.e., tertiary, and quaternary structures) may also serve as scaffolds. One example is the collagen helix. Double stranded DNA, Holliday junctions, and RNA hairpins are non-limiting examples of polynucleotide scaffolds.
One embodiment of a method for forming arrays of metal, alloy, semiconductor and/or magnetic clusters involves placing a scaffold on a substrate, in, for example, a predetermined pattern. Arrays are formed by contacting the scaffold with plural, monodispersed (clusters of substantially the same size) ligand-stabilized metal, alloy, semiconductor and/or magnetic clusters that couple to the scaffold. If the clusters are metal clusters, then the metal may be selected from the group consisting of Ag, Au, Pt, Pd and mixtures thereof. If gold is the metal, the metal cluster may be Au55.
Clusters may be coupled to a scaffold by ligand exchange reactions. Each cluster, prior to contacting the scaffold, includes plural exchangeable ligands bonded thereto. The ligand-exchange reactions involve exchanging functional groups of the scaffold for at least one of the exchangeable ligands of the cluster that is present prior to contacting the scaffold with the clusters. Examples of exchangeable ligands suitable for forming metal clusters in accordance with the invention may be selected from the group consisting of thiols, thioethers (i.e., sulfides), thioesters, disulfides, sulfur-containing heterocycles, 1xc2x0, 2xc2x0 and perhaps 3xc2x0 amines, pyridines, phosphines, carboxylates, nitrites, hydroxyl-bearing compounds, such as alcohols, and mixtures thereof. Thiols are particularly useful ligands for practicing the present invention.
Clusters may also be coupled to the scaffold by electrostatic interactions between the cluster and the scaffold. For example, clusters may include plural ligands that possess a charge or charges, either positive or negative, that serve to attract the clusters to oppositely charged scaffolds. In one embodiment, the cluster includes ligands having at least one positive charge and the scaffold is a polynucleotide having plural negative charges along its phosphate backbone. In a more particular embodiment, the cluster includes ligands having quaternary ammonium groups. In another embodiment, the cluster includes ligands with at least one negative charge, such as ligands having carboxylate or sulfonate group(s), and the scaffold is a polypeptide, such as polylysine (PL), having plural positive charges. In a particular disclosed embodiment, the scaffold is poly-L-lysine (PLL).
Clusters may be coupled to a scaffold through hydrophobic interactions. In one embodiment, the cluster includes ligands with a portion that can intercalate into DNA. For example the portion may be an anthraquinone. Other examples of suitable intercalating portions include planar cations such as acridine orange, ethidium, and proflavin. In some embodiments, the portion facilitates intercalation at particular, sequence-specific sites within a DNA molecule. In other embodiments the clusters are coupled to a scaffold through covalent bonds between the ligands of the cluster and the scaffold.
There are several methods for placing a scaffold onto a substrate in predetermined patterns. For example, one method comprises aligning scaffold molecules in an electric field created between electrodes on the substrate. It therefore will be appreciated that the scaffold molecules advantageously have a dipole moment sufficient to allow them to align between the electrodes. This is one reason why polypeptides that form a helices are particularly useful. The xcex1-helix structure imparts a sufficient dipole to the polypeptide molecules to allow alignment of the molecules between the electrodes upon formation of an electrical field. One example of a polypeptide useful for forming scaffolds in accordance with the present invention is polylysine. Similarly, polynucleotides, such as DNA, that assume helical structures may be aligned by electric fields.
Another method of patterning scaffold molecules comprises polymerizing monomers, oligomers (10 amino acids or nucleotides or less), or small polynucleotides or polypeptides into longer molecules on the surface of a substrate. For example, scaffold molecules can be polymerized as a bridge between electrodes on a substrate.
Yet another method of placing a scaffold onto a substrate in a predetermined pattern is by anchoring the scaffold and inducing alignment of the anchored scaffold in a particular direction by fluid flow. For example, a scaffold may be aligned between two electrodes by attaching the scaffold to a first electrode and using fluid flow in the direction of a second electrode to align the scaffold with the direction between the two electrodes. In a particular embodiment, the substrate is mica, the scaffold is DNA, and the DNA is attached to the first electrode using a thiol linkage. Fluid-induced alignment is used to align the scaffold in the direction of the second electrode, and the DNA scaffold is bound to the mica substrate by Mg2+ ions, thereby holding the DNA in its aligned position. Fluid-induced alignment may also be subsequently used to align additional scaffolds so that they cross, or intersect scaffolds already aligned on the substrate.
Other methods of placing a scaffold onto a substrate in a predetermined pattern include positioning the scaffold on a substrate using magnetic fields, optical tweezers, or laser traps. Multiple scaffolds may be arranged on a substrate using any of the above methods. Scaffolds may not only be aligned between electrodes but may also be aligned such that they cross or otherwise contact each other to form one-, two- or three-dimensional structures useful as templates for forming electronic devices comprising cluster arrays. Such cluster arrays may be used to provide high density electronic or memory devices that operate on the principle of Coulomb blockade at ambient temperatures.
The present invention also provides compositions that are useful, for example, for forming metal, alloy, semiconductor and/or magnetic cluster arrays. In a particular embodiment, the composition comprises monodispersed, ligand-stabilized Au55 metal clusters coupled to a polypeptide in the shape of or capable of forming an xcex1-helix with the metal clusters bonded thereto. In another embodiment, the composition comprises monodispersed, ligand stabilized gold metal clusters coupled to a polynucleotide capable of forming a helical structure. In particular embodiments, the metal clusters have metal-cluster radii of from about 0.4 nm to about 1.8 nm, such as from about 0.4 nm to about 1.0 nm.
In particular embodiments, the present invention includes compositions comprising a polypeptide capable of forming xcex1-helix and plural monodispersed, ligand-stabilized metal and/or semiconductor clusters, each cluster having plural ligands that serve to couple the clusters to the polypeptide. In more particular embodiments, the plural ligands of the clusters interact with the polypeptide by an interaction selected from the group consisting of ligand exchange reactions, electrostatic interactions, hydrophobic interaction, and combinations thereof. In other more particular embodiments, the metal and/or semiconductor clusters have radii of from about 0.4 nm to about 1.8 nm, such as between about 0.4 nm and about 1.0 nm. If the clusters comprise metal clusters the metal may be selected from the group consisting of Au, Ag, Pt, Pd and mixtures thereof, and in particular embodiments the clusters may comprise Au55 metal clusters.
Compositions comprising polynucleotides capable of forming helical structures and plural monodispersed, ligand-stabilized metal and/or semiconductor clusters, where each cluster having plural ligands serves to couple the clusters to the polynucleotide are also provided by the invention. The plural ligands of the clusters may serve to interact and couple the cluster to the polynucleotide through interactions such as ligand exchange reactions, electrostatic interactions, hydrophobic interactions, intercalation reactions and combinations thereof.
In particular embodiments the invention provides organized arrays of metal clusters comprising monodispersed, ligand-stabilized metal clusters having metal-cluster radii of from about 0.4 nm to about 1.8 nm, the metal being selected from the group consisting of Ag, Au, Pt, Pd and mixtures thereof. Such arrays include a scaffold and the metal clusters are coupled to the scaffold to form the organized array.
The present invention further provides an electronic device that operates at or about room temperature based on the Coulomb blockade effect. Such electronic devices include a first cluster (e.g. a cluster comprising a metal cluster core having a radius of between about 0.4 nm and about 1.8 nm) and a second such cluster. The clusters are physically spaced apart from each other at a distance of less than about 5 nm by coupling the clusters to a scaffold, such as a biomolecular scaffold, so that the physical separation between the clusters is maintained. Electronic devices according to the invention may also include pairs of biomolecular scaffolds, each with coupled clusters, arranged so that the scaffolds intersect to provide electric circuit elements, such as single-electron transistors and electron turnstiles. Such elements may be useful as components of chemical sensors or ultrasensitive electrometers. Because of their unique architecture, electronic devices according to the invention exhibit a linear increase in the number of electrons passing between pairs of clusters as the potential difference between the two clusters is increased above a threshold value.
The present invention also provides methods of forming monodispersed phosphine-stabilized gold nanoparticles that allow the radii of nanoparticles to be controllably adjusted. In one embodiment, the method comprises dissolving HAuCl4 and PPh3 in a biphasic system (for example, a biphasic system comprising a water phase, an organic phase, and a phase transfer catalyst) and adding sodium borohydride to the biphasic system. In particular embodiments, the biphasic system may comprise water and an organic solvent selected from the group consisting of toluene, xylenes, benzene, and mixtures thereof. The phase transfer catalyst may be a quaternary ammonium salt, for example, tetraoctylammonium bromide. Control of the nanoparticle size may be accomplished through control of the rate at which sodium borohydride is added to the biphasic system.
The invention also provides methods of preparing thiol-stabilized gold nanoparticles. Thiol-stabilized gold nanoparticles may be prepared by dissolving phosphine-stabilized gold nanoparticles in an organic solvent and exchanging the phosphine ligands of the phosphine-stabilized gold nanoparticle for thiol ligands. Particles that are particularly useful for preparing arrays according to the invention are prepared from thiol ligands that comprise a group or groups of atoms that are capable of coupling thiol-stabilized gold nanoparticle to scaffolds. Phosphine and thiol ligands may be prepared in a single-phase system if the thiol ligand is soluble in an organic solvent. However, if the thiol ligand(s) is water soluble, it is still possible to exchange thiol ligands for phosphine ligands at the interface between a water-immiscible organic solvent containing the phosphine stabilized gold nanoparticles and water containing the thiol ligand(s).